1. Field of the Invention
The invention relates to the supply of operating voltages for mass spectrometers, particularly Kingdon electrostatic ion traps and time-of-flight mass spectrometers.
2. Description of the Related Art
The term Kingdon ion traps refers to electrostatic ion traps in which ions can orbit around one or more inner longitudinal electrodes, or oscillate between inner longitudinal electrodes, and where an outer, enclosing housing is at a DC potential which the ions with a specified kinetic energy cannot reach. A very simple Kingdon ion trap consists of a rod (ideally an infinitely long rod) as the inner electrode and a surrounding tube as the housing or outer electrode. In special Kingdon ion traps which are suitable for use as mass spectrometers, the inner surfaces of the housing electrodes and the outer surfaces of the inner electrodes are designed so that, firstly, the motions of the ions in the longitudinal direction (z) of the Kingdon ion trap are decoupled from their motions in the transverse direction (x, y) or (r, φ) and, secondly, a parabolic potential profile is generated in the longitudinal direction, in which the ions can oscillate harmonically. The frequencies of these oscillations can be determined from the measured image currents by means of Fourier transforms; they represent a measure for the mass of the ions.
In this publication, the terms “Kingdon ion traps” and “Kingdon mass analyzers” refer in particular to these special forms in which ions can oscillate harmonically in the longitudinal direction, decoupled from their motions in the transverse direction.
Different types of Kingdon ion trap with these characteristics are known. The patent specification U.S. Pat. No. 5,886,346 (A. A. Makarov) describes the fundamentals of a special Kingdon mass analyzer which was launched by Thermo-Fischer Scientific GmbH Bremen under the name Orbitrap®. The electrostatic ion trap here consists of a housing electrode which is split across the center and a single spindle-shaped coaxial inner electrode (FIG. 1). The housing electrode has an ion-repelling electric potential and the inner electrode an ion-attracting electric potential. The cross-sections of the inner surface of the housing electrodes and the outer surfaces of the inner electrodes are both circular. With the aid of a special ion-optical device and a special injection method, the ions are tangentially injected through an opening in the housing electrode and then orbit in the hyperlogarithmic electric potential of the ion trap. The kinetic injection energy of the ions is set so that the centripetal attractive forces and the centrifugal forces are in balance, and the ions therefore largely move on practically circular trajectories.
In the document DE 10 2007 024 858 A1 (C. Köster), other types of Kingdon ion trap are described which, in one preferred embodiment, have precisely two inner electrodes (FIG. 2). Here too, the inner electrodes and the outer housing electrodes can be precisely shaped in such a way that a potential distribution is formed in which the longitudinal motions are decoupled from the transverse motions, and a parabolic potential well is created in the longitudinal direction to generate a harmonic oscillation. These “bipolar Cassini ion traps” or “second-order Cassini ion traps” are characterized by the fact that the ions not only can fly on complicated trajectories around the two inner electrodes, but can also oscillate transversely in the center plane between the two inner electrodes. The orbiting or transversely oscillating ions can then execute harmonic oscillations in the longitudinal direction.
All these Kingdon ion traps can be used as mass analyzers by measuring the image currents induced by the axial oscillations of the ions in bisected housing electrodes (or bisected inner electrodes), and by processing them appropriately with the aid of Fourier transforms to produce mass spectra. They therefore belong to the class of Fourier transform mass spectrometers (FT-MS). The electrodes can also be divided into more than two insulated partial segments in order to detect higher-order oscillations. The electric fields in the interior are generated by voltages between the inner and outer electrodes; the voltages are regularly between two to eight kilovolts, but can also be selected to be much higher. The higher the voltage, the higher will be the mass resolution and mass accuracy. This voltage is called “operating voltage” here; it determines the frequency with which the ions of a specific mass oscillate, and thus has a “frequency dispersive” effect and therefore a “mass-dispersive” effect, in contrast to lens voltages or voltages at beam deflectors for example.
In practical operation, the image currents are measured over a period of 0.1 to 10 seconds, depending on the analytical task; usually between 0.2 and 1 second in order to obtain several mass spectra of the substance peaks when the device is coupled to substance separators such as chromatographs. The longer the measured image current transient, the higher the mass resolving power.
As is known from ion cyclotron resonance mass spectrometers (ICR-MS), Fourier transform mass spectrometers provide extremely high mass resolutions and mass accuracies. This also applies to Kingdon mass spectrometers, but only if the ions can oscillate in the axial direction in an electric field with high temporal constancy. The electric field in Kingdon ion traps has an equivalent function to the magnetic field of the ICR mass spectrometers. The magnetic field of the ICR mass spectrometers is usually generated by superconducting solenoids, which have magnetic field strength decreases of only around 10−8 per day (≈10−13 per second). Moreover, these magnetic fields have no superimposed noise or hum; even magnetic storms of the Earth's magnetic field are shielded.
Such temporal constancy and freedom from noise of the DC operating voltage for the electrostatic fields in Kingdon ion traps cannot be achieved with today's high-voltage generators. These high-voltage generators are usually based on the active switching of voltages and currents, which cause high voltages to be induced in the secondary winding of a transformer, and they have feedback control circuits (i.e. active regulation) which unavoidably generate small control oscillations. High-voltage generators comprising means for active switching of voltages and currents and/or feedback control circuits may be termed active high-voltage generators. Top-quality active high-voltage generators, specifically designed for these purposes, have a short-term constancy of around 10−6 per minute; often this is dependent on ambient temperature and supply voltage, albeit only slightly. Drifts of the operating voltage may occur toward both larger and smaller voltages, so they cannot be corrected prospectively. Even more serious, however, are voltage variations (ripple) due to control oscillations of the order of magnitude of 10−6 or more, sometimes also exhibiting superimpositions of different frequencies (residuals of the supply voltage frequency, for example). The voltage variations by ripples are on a short time scale, in particular on a time scale for acquiring a single mass spectrum or few subsequent mass spectra.
Noise-free DC operating voltages which are constant over time are required not only in Kingdon mass spectrometers, but also in other mass spectrometers. A time-of-flight mass spectrometer with orthogonal injection of the ions into an ion pulser, as is shown schematically in FIG. 5, is given here as a second example. The ion pulser usually pulses sections of a continuous ion beam, at right angles to the direction of flight of the ions, into an acceleration lens system, which accelerates the ions into the flight region of a time-of-flight mass spectrometer with a reflector. The acceleration is carried out with voltages of 10 to 20 kilovolts. This accelerating voltage determines the mass-dependent time of flight of the ions; it therefore has a “time-of-flight dispersive” effect The pulse rate of the ion pulser is usually 5 to 10 kilohertz, which means that 5000 to 10,000 time-of-flight spectra are acquired in one second, which are summed over a specified duration of 0.05 to 20 seconds, subjected to a peak recognition algorithm and then converted into a mass spectrum. The accelerating voltage is applied to an enclosure of the flight region, which is located within a grounded housing. The decelerating voltages on the individual diaphragms of the reflector must also be as free from drift, noise and ripple as possible when high mass resolutions and mass accuracies are required. Today, top-class time-of-flight mass spectrometers are designed to achieve mass accuracies which are far better than a millionth of the mass.
It should be noted here that slight, but constant, drifting of the DC operating voltages having a critical mass-dispersive effect, can be compensated by mathematical methods; but the noise cannot, and ripple only to a very limited extent. Voltage drifts can be corrected during the acquisition of an image current transient in a Kingdon mass analyzer: see the documents DE 10 2008 025 974 B3, GB 2 461 965 A or U.S. Pat. No. 7,964,842 B3 (C. Köster and K. Michelmann, 2008). Voltage drifts nevertheless have a negative effect because the analytical method then drifts away from its mass calibration, making it necessary to take special measures, such as the use of internal mass references. If the drifts always occur in the same direction and are very constant, however, then the resulting changes in the operating voltages can be automatically taken into account in mass calculations from the image current transients or the times of flight.